1. Field of the Invention
The present invention relates to an electric power steering apparatus using a motor as a generating source of steering assistance force and a method of manufacturing a gear used for the electric power steering apparatus.
2. Description of Related Art
An automobile is steered by transmitting rotating operation of a steering wheel disposed at the interior of passenger's room to a steering mechanism disposed at the exterior of the room for turning tire wheels for steering (front wheels, in general).
FIG. 1 is a cross-sectional view showing a constitution of a conventional electric power steering apparatus.
The figure shows an example of an electric power steering apparatus for an automobile, comprising: a first steering shaft 101 connected to a steering wheel 100 for steering; a second steering shaft 103 connected at one end thereof via a torsion bar 102 to said steering shaft 101 coaxially and at the other end thereof to a steering mechanism joined to tire wheels; a torque sensor 104 for detecting a torque applied on the first steering shaft 101 with rotation of the steering wheel 100 based on torsion generated on the torsion bar 102; a steering assist motor 105 to be driven based on the result of detection of said torque sensor 104; and a reduction mechanism joined to an output shaft of said motor 105 for reducing rotation of said output shaft and transmitting the reduced rotation to the second steering shaft 103. Said reduction mechanism has a worm 106 and a worm wheel 107. The electric power steering apparatus is constituted to lighten driver's load for steering by assisting the action of the steering mechanism corresponding to rotation of the steering wheel 100 with rotation of the motor 105.
The worm 106 constituting the reduction mechanism is supported at the interior of a housing 108 via a pair of antifriction bearings (not illustrated). The second steering shaft 103 provided with the worm wheel 107 is supported at the interior of the housing 108 via a pair of antifriction bearings 109, 109.
The worm wheel 107 comprises a ring-shaped tooth body 110 made of synthetic resin which has teeth meshing with the worm 106 and a support body 111 made of metal engaged with the interior side of said tooth body 110. The tooth body 110 made of synthetic resin reduces a jarring noise made by meshing with the worm 106 and realizes high workability of the teeth. A plurality of locking grooves 114 (see FIG. 2) are provided on a peripheral surface of the support body 111, and prevents relative rotation of the tooth body 110 and the support body 111.
FIG. 2 is a cross-sectional view showing a constitution of a conventional support body. The support body 111 of the worm wheel 107 is formed through cut processing as in FIG. 1 or through cold forging as in FIG. 2. The support body 111 formed through cold forging comprises: a cylindrical part 112 engaged with the tooth body 110; a plate part 113 extended inward in a radial direction from one end of said cylindrical part 112; and the locking grooves 114 provided on the peripheral surface of the cylindrical part 112. The support body 111 is integrally united with the tooth body 110 by placing the support body 111 in a mold for injection molding and performing injection molding.
However, when the support body is formed through cut processing and cold forging, there arises a problem that unnecessary deposit cannot be removed enough. The support body with great weight enlarges inertial force at the time of steering, and causes a deterioration of feeling of steering. In addition, cut processing and cold forging were disadvantageous in comparison with press forming from the viewpoint of manufacturing costs.
The present applicant has applied an electric power steering apparatus a support body of which is press-formed (Japanese Patent Application Laid-Open No. 2001-206230).
The engagement part of the press-formed support body with the tooth body comprises a disc part having a plurality of through holes to be filled with synthetic resin for molding the tooth body which penetrate in an axial direction and a curved edge part provided with a plurality of locking grooves which is curved in an axial direction from an exterior edge of said disc part.
A remedy has been desired for solving the following problems of the press-formed support body. One problem is that the support body is complicated in structure since a relative slide of the tooth body and the support body is hindered at two positions which are the through holes and the curved edge of the engagement part. Another problem is that circularity of synthetic resin in the mold for injection molding at the time of molding the tooth body is low since the synthetic resin for molding the tooth body is filled into the through holes of the engagement part, which results in generation of voids and weld marks.
In order to simplify a structure of the engagement part of the press-formed support body, a support body without the through holes is conceivable. Such a support body should have a curved edge which is long in an axial direction provided with locking grooves on the peripheral surface thereof to ensure an enough contact area with the tooth body. However, manufacture of said support body requires use of a relatively expensive and large presser to perform plastic deformation exclusively on the locking groove part on the peripheral surface of the support body. Consequently an amortization expense of the presser is added to a unit price of the support body, and the unit price of the support body amounts to approximately the same as that of the support body provided with the through holes.